In some kinds of gas combustion type driving tools, combustible gas is injected into a combustion chamber sealed within a body, then mixed gas of the combustible gas and the air is stirred within the combustion chamber, and the mixed gas thus stirred is burnt within the combustion chamber to generate high-pressure combustion gas within the combustion chamber. The high-pressure combustion gas acts on a striking piston housed within a striking cylinder to impulsively drive the striking piston within the striking cylinder. A nail supplied to a nose portion beneath the body is driven into a steel plate or concrete by a driver coupled to the lower surface side of the striking piston. Such the combustion-gas driven driving machine is formed as a portable tool in a manner that a vessel such as a gas container filed with the combustible gas is attached within the tool and a battery acting as a power source for igniting the combustible gas is attached to the tool. Thus, the combustion-gas driven driving machine can perform the driving procedure of a nail or a pin without being restricted by a source of drive such as electric power or compressed air.
In the aforesaid gas combustion type driving tool, the striking piston is provided so as to be slidable in an up/down direction within the striking cylinder disposed within the body. The movable sleeve fitted on the outside of the striking cylinder so as to be movable in the up/down direction moves upward and abuts against a cylinder head provided above the striking cylinder to seal a space, whereby the combustion chamber can be configured. The movable sleeve is interlocked with a contact arm mechanism serving as a safety device. When the tip end of the contact arm is pressed against a member to be driven and is moved upward relatively with respect to the body, simultaneously the movable sleeve moves upward, whereby the combustion chamber sealed between the cylinder head and the striking cylinder is formed. When the contact arm restores to the original state, the movable sleeve also moves downward to open the combustion chamber.
The pulling operation of a trigger is made possible for the first time after the contact arm is pressed against the driven member and so the combustion chamber is formed. Further, if the trigger is not kept to be pulled, the movable sleeve moves downward and so the sealed state of the combustion chamber can not be held. JP-A-2004-074296 discloses an example of such a gas combustion type driving tool.
However, in the case where the trigger is released immediately after the trigger is pulled to drive the tool or in the case where the body separates instantaneously from the driven member due to the reaction at the time of the driving operation, there may arises a case that the contact arm also moves downward due to the urging force of a spring and hence the sealing state of the combustion chamber is lost. If the combustion chamber is opened at the early timing, since the pressure within the combustion chamber is not reduced sufficiently, there arises a phenomenon that the striking piston cannot return to the top dead center. In this case, the striking force at the next striking operation becomes insufficient and so a fastener rises from the driven member disadvantageously.